clstr2tree: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/clstr2tree - from package: cd-hit

You may install the required package with command apt-get install {package}

Error in other languages:
clstr2tree: bevel nie gevind nie
clstr2tree: comandă negăsită
clstr2tree: comando não encontrado
clstr2tree: commande introuvable
clstr2tree: command not found
clstr2tree: käsku ei ole
clstr2tree: không tìm thấy lệnh
clstr2tree: komanda nerasta
clstr2tree: Kommando nicht gefunden.
clstr2tree: kommandot finns inte
clstr2tree: komut yok
clstr2tree: nie znaleziono polecenia
clstr2tree: níor aimsíodh an t-ordú
clstr2tree: no se encontró la orden
clstr2tree: no s'ha trobat l'ordre
clstr2tree: opdracht niet gevonden
clstr2tree: parancs nem található
clstr2tree: perintah tidak ditemukan
clstr2tree: príkaz nenájdený
clstr2tree: příkaz nenalezen
clstr2tree: команда не найдена
clstr2tree: командата не е открита
clstr2tree: 命令找不到
clstr2tree: コマンドが見つかりません